Discounted Price for Products Under 50

The problem

Brightlane's promotions team is configuring a flash sale that applies a 10% discount to all products priced under $50.

Write a query to return the name, original price, and discounted price for each qualifying product.

Assumptions:

The products table contains every item in Brightlane's catalog.
The price column records each product's regular selling price in dollars.
The discounted price is the original price multiplied by 0.9 (a 10% reduction).

Output:

One row per qualifying product, with columns name, price, and discounted_price.

Schema · ecommerce 5 tables

categories

id integer

name text

parent_id? integer

products

id integer

name text

category_id integer

price numeric

stock_qty integer

attributes? jsonb

order_items

id integer

order_id integer

product_id integer

quantity integer

unit_price numeric

customers

id integer

name text

email text

city? text

country text

created_at timestamptz

is_active boolean

orders

id integer

customer_id integer

ordered_at timestamptz

status text

total_amount numeric

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Worked solution Try it yourself first

Solution query

SELECT
  name,
  price,
  price * 0.9 AS discounted_price
FROM
  products
WHERE
  price < 50

The shape

Two independent jobs in one query — WHERE decides which products qualify for the flash sale, and SELECT builds the discounted price for the ones that do.

Clause by clause

SELECT name, price, price * 0.9 AS discounted_price returns three columns per surviving row. The first two come straight from the table; the third is computed on the fly by multiplying price by 0.9, which is the same as taking 10% off. The AS alias labels the new column so the result reads as a domain quantity instead of ?column?.
FROM products is the source — every item in Brightlane's catalog.
WHERE price < 50 filters down to the products under $50. Strict < excludes the boundary, so a product priced at exactly $50 would not qualify. The filter runs against the raw price column, not the discounted one.

Why the filter uses `price`, not `discounted_price`

The alias discounted_price doesn't exist yet at the time WHERE runs. PostgreSQL evaluates FROM first to establish the population of rows, then WHERE to filter, and only later does SELECT produce the output columns and assign their aliases. By the time discounted_price is a name, the filtering is over. Trying to write WHERE discounted_price < 45 would raise an error: the column doesn't exist at that stage of the query. The filter has to be written against the source column the table actually has.

You practiced filtering rows AND computing a derived column in the same query. WHERE decides which rows go through, SELECT decides what each row looks like — they're independent layers.

Return the name, original price, and discounted price for each qualifying product

The shape

Clause by clause

Why the filter uses `price`, not `discounted_price`

Reading explains SQL. Writing it, over and over with instant feedback, is what makes you fluent.

The shape

Clause by clause

Why the filter uses price, not discounted_price

Reading explains SQL. Writing it, over and over with instant feedback, is what makes you fluent.

Why the filter uses `price`, not `discounted_price`